1. Field of the Invention
The present invention relates to a method for manufacturing a elongated welded clad steel tube having excellent heat and corrosion resistances as well as good bending property.
2. Description of the Related Art
Waste heat obtained from burning inflammable industrial waste and municipal waste have recently come to be utilized in power generation for the effective use of energy. Heat transfer boiler tubes used in power generation are made of steels whose grades according to the Japanese Industrial Standards (JIS) are STB 340 and STBA 24. Since exhaust gas resulting from combustion brings about problems of corrosion by the molten salts contained in adhering ashes and/or highly corrosive gases such as chloride and sulfide gases, the steam temperature have to be reduced to 300.degree. C. or less. Unfortunately, the maximum power generating efficiency of the conventional method is about 15%. In order to overcome this problem, attempts have been made to increase the power generating efficiency to 30% by raising the steam temperature to 500.degree. C.
It has been proposed to use a Ni alloy, such as Alloy 625 (22% Cr-62% Ni-9% Mo), as a heat transfer tube for power generating boilers whose operating stem temperature is 500.degree. C. or more. The heat and corrosion resistance of the Ni alloy is extremely greater than STB 340 and STBA 24. Further, the following methods have been also proposed. A method of coating a Ni alloy or the like having excellent heat and corrosion resistance to the outer periphery surface of a currently used steel tube by powder spraying. A method of overlapping a Ni alloy or the like having excellent heat and corrosion resistance on the outer periphery surface of a currently used steel tube by plasma powder overlapping.
However, the method in which a Ni alloy having excellent heat and corrosion resistance, e.g., Alloy 625, is used as a material of the heat transfer tube for power generating boilers addresses the problem that the Ni alloy is expensive. The method in which a Ni alloy having excellent heat and corrosion resistance is coated to the outer periphery surface of the currently used tube by powder spraying addresses problems, one of which is that corrosion resistance is impaired due to overlay thickness, weld strength, and microvoid.
Further, the method of overlapping a Ni alloy having excellent heat and corrosion resistance on the outer periphery surface of a steel tube by plasma powder overlapping addresses no problems with respect to overlay thickness, weld strength, and heat and corrosion resistance, however not only has a shortcoming in obtaining accurate thickness, but also requires a large-sized welding machine for manufacturing elongated tubes, which entails an enormous cost.
Still further, having large surface roughness as overlaid, the clad steel tube obtained by this method also imposes the problem of poor nondestructive inspection accuracy.
Still further, there has been provided no appropriate heat transfer tubes for black liquid recovering boilers used in the paper manufacturing industry which have excellent heat and corrosion resistance as well as bending property and which are also inexpensive.